When sizing 308 down to 260, the case neck goes from .308 to .264. As you size the neck down, the brass that was somewhere around .011" to .013" thick at .308", gets squeezed down also and the case necks become much thicker. When you seat a bullet in this sized down case, the case neck is too thick with a bullet seated, and the case neck will now hit the chamber walls of the 260. To fix it, you have to neck turn the case necks to remove some thickness. Can you use 308 brass to form 260, yes, but you will have to neck turn the necks to make it work.
Win 7-08 brass has worked well to size down to 260 (plus gives you a little added case volume over Rem and Lapua cases). I have also used Lapua 243 Win brass to neck up 260. Either of these options will work without any more case prep.